Scrambled television



oct. 11 1960 W. J-. SHANAHAN SCRAMBLED TELEVISION Filed Aug. 25, 1954 2Sheets-Sheet 1 R m w m WILL/A BY- ai auf z n N m M w A H *www Oct. l1,1960 w. J. sHANAHAN SCRAMBLED TELEVISION 2 vSheets-Sheet 2 Filed Aug.25, 1.954

w/LL/AM .1 SHA /vA HA/v WMM/f ATTORNEYS United States S'CRAlVIBLEDTELEVISION Filed Aug. 25, 1954, Ser. No. 452,095

2 Claims. (Cl. 1785.1)`

This invention relates to scrambled television tr-ansmission methods andsystems, and transmitters and receivers therefor. 'A

It has been proposed in the -art to which this invention pertains toperiodically invert the video signals as to blacks and Whites anddegrees thereof in relation to the modulation of the transmitted signal,to cause distortion in the display of an unauthorizedreceiver. Thisinvention pertains particularly to such inversion of the video signalswith the further provision of signals to prevent certain effectsinherent in the human eyesfrom establishing at least a sense of thetelevised picture.

Basically, the present invention contemplates the periodic inversion ofvideo signals together with the insertion of a spurious signal such as anoise signal at predetermined intervals, the -arrangement being suchthat an unauthorized receiver, that is, one designed 'for normaltelevision reception, will present a display so ldistorted as to beunrecognizable, while an authorizedreceiver will present an entirelysatisfactory and undistorted picture.

Accordingly, it is a primary object of this invention to provide animproved television scrambling method,

system and apparatus.

It is a further object of this invention to provide a method andapparatus whereby video signals in Ia television transmission may beperiodically inverted and wherein spurious signals may be inserted intothe transmission to completely distort the display of an unauthorizedreceiver.

Further objects and the entire scope of the invention will become morefully apparent from the: hereinafter detailed description ofillustrative embodimentsV of the invention, and from the appendedclaims.

The invention may be best understood with reference to the accompanying-drawings whichY show illustrative embodiments and other descriptivematter las follows:

Figure 1 in parts a through finclusive shows a vertical bar test patternand accompanying radio frequency envelopes and detected waveforms ofvideo signals` and a spurious signal.

Figure 2 shows a circuit for applying spurious'signals into a televisiontransmission and inverting video at the transmitter, or reinventingvideo `and suppressing spurious signals at the receiver end of atelevision system according to the present invention.

Figure 2A in parts a through c inclusive shows waveforms applicable toFigure 2.

Figure 3 shows pertinent components of a television transmitterutilizing the circuit of Figure 2 according to the present invention.

Figure 4 shows a television receiver according to the present inventionequipped to receive transmissions from the transmitter of Figure 4 forunscrambled display, and

Figure 5 shows a modification of thek circuit Aof Figure 2.

Part a ofV Figure 1 is intended to illustrate a test pattern on which atelevision camera may be trained, or

arent ice the display'of the test patternon `an authorized receivertuned to receive the television transmission. The rectangle shown inthis pant of Figure 1 is intended to represent a grey background havinga vertical black bar located centrally'thereof. Part b of Figure 1 isintended to show the radio frequency carrier of the televisiontransmission during a single line trace across the pattern of pant a,the amplitude of the radio frequency showingthe grey level and the blacklevel, it being understood that the-y transmission is such that theblack level is in the direction Vof the maximum signal amplitude andthewhite level near the minimum amplitude. Part c of Figure 1 shows thecase for inverted video signals, the amplitude of the carrier frequencyduring the interval` of the black bar in the pattern decreasing towardwhite instead of increasing toward' black (the latter being shown` inpart b). Part d of'Figure l shows the rectied radio Y carrier frequencyof a transmission according to part b,

and part e shows the rectifiedcarrier signal according t0 transmissionshown inpart` c. Partv f of Figure-1 isjintended to representa detectedvideo signal madefup of a random or noise signal.

`As has been suggestedhereinabove, it is found; that when video signalsare regularly, inverted, that is, invertedline'` for line, *field foreld, etc., the retentivi-ty of thehuman eyes not merely tends tointegrate Yout the blacks and: whiteV which` 4arecaused` by ablackVobject such as the black bar in the test pattern-now lseingiAdiscussed, but actually tends to-create a white bar.` VThis is truebecause Vthe human eye appears to .be non-.linear V'and isl moreresponsive to increases inbrightness.. than todecreases in brightness.Therefore, evenftheregtdar inversion of` video will not serve toentirelyy 'obscure the picture.V .Howeven :according tothe presentinvention Vit has been found that whena spurioussignal suchasanoisesignal is introduced to repl-aceone inl every so many the regularinversionof video linerforV line, frarne for frame, etc. Additionally,it is believed to-bea preferred Ymode of operation toinvert Yvideo atleast lineforgline,

rather thanlield fofrieldv or longerV periods of time;

It is thought to be apparent that where aftransmission is createdaccording -to the above described Vmethod;Jan unauthorized televisionreceiver hav-ing no means there-in for inversionrofthevideoorcancellation of lines offnois, will show on the`displayjidevicein the exampleof line forjline inversion, a picturewhich'is somewhat cancelled-outro show a grey level, but with one ofevery n lines, say every sixth line', avnoise or randomsignal whichserves to obscure therpieture which the eyesmay tend to reconstruct dueto thek non-linear characteristics discussed aboveu f* f Y A I Itisnecessary'that authorized receivers be equipped to reinvert theinverted signals asrequired, andeliminate vthel spurious lines so as toproperlyreconstruct thepic- 2,95e,11o Y 3 flop circuits A and B, acount-down circuit 14, a noise source circuit 16, gate circuits 18, 20,22, 24 and 26, and a signal inversion circuit 28.

The lijp-flop circuits A and B may be conventional Eccles-Jordan typecircuits having two inputs and two outputs and shiftable between twostable states so that in one stable state one of the output leads willcarry a relatively high potential and the other output lead a relativelylow potential, these potential conditions being reversed when thecircuit is shifted to the other stable state. For example, the circuit Amay be such that when a negative pulse is applied to the l input 30, arelatively high potential will appear on the l output line 32 (unlessthat condition previously existed). Assuming a shift to a relativelyhigh potential on output line 32, a negative pulse on input line 314will change the potential on the 0 output line 36 from the relativelylow to the relatively high potential. The gate circuits may includevacnum tubes having a number of control grids commensurate with thenumber of input lines, so that a desired change of potential on theoutput line exists only during the occurrence of input signals. Properinversion circuits (not shown) are to be associated with each gate toinsure proper functioning. The count-down circuit 14 may be any one of anumber of well known counting circuits such as a series ofinterconnected Hip-flop circuits. I'he noise source 16 may be anyconvenient source of random or regularly recurring signal. It ispreferable in the practice of the present invention that the amplitudeof the noise signal be at least as great as the peak to peak variationsin the video signal.

The operation of Figure 2 may be best explained with the assumption thata series of regularly recurring control pulses exist on line 38 whichforms the output of the control pulse generator 12. A plot of suchregularly recurring pulses is shown in Figure 2A, line a thereof. Thesecould be at the horizontal line pulse rate of the television system orsome other ra-te. Line 38 is connected to both the O and the l input offlip-op A and therefore each pulse will serve to shift the ip-op,whereby the relative potentials on the 0 and the 1 output lines (lines36 and 32, respectively) will shift between levels as shown in line b ofFigure 2A, one waveform being shown for convenience for demonstratingthe relative potentials on both lines. The 0 output of ip-ilop A servesas one enabling input to gate 22 and the 1 output of llip-tlop A servesas one enabling input to gate 24. When the ilipflop A is in its 1 state,gate 24 will be opened and when in its 0 state gate 22 will be opened.

Video signals are applied to line 40 and are passed through gate 20 andapplied to gates 22 and 24. Video signals passing through gate 22 areapplied to output line 42, while video signals passing through gate 24are inverted in the inverter circuit 28 and then applied to line 42.Assuming that the control pulses on line 38 (line a of Figure 2A) occurat the horizontal line repetition rate, a irst line of video signalswill pass through gate 22 without inversion whenever this gate is openedby flip-flop A, and the next line of video signals will be inverted andappear on line 42 having passed through gate 24 which is opened whilegate 22 is closed. Thus, video signals on line 42 are inverted at thehorizontal line rate.

The flip-flop B has its 0 output connected as the enabling input to gate20, and the 1 output is connected as the enabling input to gate 18 andalso to gate 26. The control pulses on line 38 are applied over branchline 44 to the countdown circuit 14. Assuming that the countdown circuit14 produces one output pulse for every eight input pulses, pulses atthis rate are applied to the 1 input of flip-flop B over line 46. Theshifting of flip-flop B to its 1 state serves to open gate 18 to permitthe next occurring control pulse to pass therethrough and to be appliedto the 0 input of tlip-op B over line 48. This pulse serves to shiftip-ilop B to its 0 state. It will now be apparent that the voltagecharacteristics of the 1 and 0 outputs of Hip-flop B are according tolline c of Figure 2A. While the flip-flop B is in its 0 state (the 0output having the high potential) gate 20 will be opened and the videosignals will ow therethrough for application to gates 22 and 24.However, during the l control pulse period while the ilip-flop B isshifted, all video signals will be blocked at gate 20, and gate 26 willbe opened. Thus, for the particular line or other period of the controlpulses, the spurious or noise signal from source 16 will be applied tooutput line 42. Y

A line 50 is shown as the input to control pulse generator circuit 12,this line carrying the code signal input. It is to be understood thatthe code signal input on line 50 may be one of a number of arrangementsfor determining the function of the exemplary so-called noise circuit10. Elaborate systems Ahave been described for periodically or randomlychanging a code situation so as to impart a great number of independentcodes into a scrambled television transmission system. For Vexample, seemy copending patent applications Serial No. 255,555, tiled November 9,1951; Serial No. 316,485, led October 23, 1952; and Serial No. 418,642,filed March 25, 1954. The control signal input on line 50 in Figure 2 ofthis application could be the waveforms on lines 4-9 inclusive of Figure2 of Serial No. 316,485, or the random or regularly recurring signals onthe lines 68, 70 and 72 of Serial No. 418,642. The control pulsegenerating circuit 12 can be among other things a set of count-downcircuits connected to a source of regular recurring pulses such as thehorizontal driving pulses, the particular output of which is to becontrolled by a set of gates, one connected to each count-down circuitand having enabling inputs determined from the code signal input lines50 by means of a conversion matrix. Assuming that the rst count-downcircuit is in a ratio of l to 2, the next in the ratio of 1 to 4, thenext in the ratio of 1 to 6, the recurrence rate of the control pulses(line a of Figure 2A) will be shifted from time to time, such as fromfield to field.

It will be further understood that the noise circuit 10 is intended tobe illustrative only and the relative occurrence of the noise signalsmay be otherwise controlled, all within the scope of the presentinvention.

Referring now to Figure 3, the transmitter equipment (exclusive ofincidental components not necessary in the present description) mayinclude a television camera 82, noise circuit 10, conventionaltransmission circuits 84 and synchronizing and code circuit 86. Thevideo signals from the camera will be applied on previously mentionedline A40 to circuit 10 and the code signal input signals will be appliedto circuit 10 over line 50. The synchronizing and code circuits 86 mayinclude means for generating the conventional horizontal and verticalcamera driving pulses to be applied to the camera over line 88 and alsoall of the code signal generating circuits as mentioned hereinabove,which may be according to those illustrative circuits in the abovementioned copending patent applications. The combined video and noisesignals will be applied over line 42 to the transmission circuits 84.

A receiving circuit may be as shown in Figure 4 where signals receivedthrough the antenna are separated as is conventional in separatorcircuits 90, the video and noise signals applied to receiver noisecircuit 10 over line 40 and coding signals applied over line 50 fromreceiver code circuits 92, the receiver horizontal and vertical drivingpulses being applied thereto over line 94. An understanding of the abovementioned copending patent applications will serve to make clear thatthe receiver code circuits 92 are essentially the complement of whatevercoding circuits are employed at the transmitt'i-Qthe general requirementbeing that thetransmitter and receiver circuits respectively havesimilar characteristics so as to maintain the display device of theauthorized receiver in step with the changes in the radiated signals.

It may be thought that the elimination of one video line or anotherportion of the display will be annoying and deteriorate the quality ofthe display. Actually, particularly in the case of the elimination ofnon-video lines in a given number of lines such as six or eight lines,the effect is hardly noticeable (While a noise display at the same ratein an unauthorized receiver serves to satisfactorily scramble thepicture). It is possible to go further to avoid distortion due to theelimination of the video by an arrangement such as shown in Figure 5. Inthis figure, in effect, the noise circuit 16 of Figure 2 is replaced bya delay line device 100 which continuously has video signals applied tothe input over line 102. The output of the delay line device is appliedto gate 26 which is thereafter connected to line 42 (the equivalent ofline 42 at the transmitter). Whenever the ip-ilop B is operated to closegate 2t) and to, in etfect, produce no video in the video display, theprevious video, say for a video line, will be repeated through gate 26by virtue of the delay in delay line 100; In the case where the video isbeing inverted at line repetition rate and the noise insertion is forthe delay of one line in n lines, the delay of the device 100 is to beequal to one line. The application of this technique Where the noiseinsertion is for more than one horizontal line, will be apparent, thedelay time having only to be commensurate with the control pulserepetition rate.

The foregoing detailed description of illustrative embodiments of theinvention has been given only for purposes of illustration, and thescope of the invention is to be determined from the appended claims.

What is claimed is:

l. In a scrambled television transmission system including atransmitting end and at least one receiving end, apparatus forscrambling the television transmission including means at thetransmitting end for inverting the transmitted video signals atpredetermined times, and means at the transmitting end for replacing thevideo signals during other predetermined times which are at leastsubstantial portions of complete line trace times with spurious signals,the receiving end including means for reinverting video signals insynchronism with said inversion at said transmitting end, means forsuppressing said spurious signals during the occurrence time thereof,and further including means at the receiving end for supplying the mostrecently available video signals in repetition to the display means ofthe receiver in lieu of said suppressed spurious signals.

2. Apparatus for receiving scrambled television transmissions in whichthe transmitted video signals are inverted at predetermined times andthe video signals are replaced during other predetermined times withspurious signals intended to be predominantly displayed, including meansfor reinverting the video signals inV synchronism with said inversion inthe transmitted signals, means for suppressing said spurious signalsduring said other predetermined times, and further including means forsupplying the most recently available video signals in repetition to thedisplay means of the receiver in lieu of said suppressed spurioussignals.

References Cited in the tile of this patent UNITED STATES PATENTS

